MFEX Rover
System and Operations Overview

Updated 8/21/96

• Rover Mission Objectives
• MFEX System Elements
  • Rover
    • Mobility
    • Power
    • Thermal
    • Communications
    • Experiments
    • Navigation
  • LMRE
  • Rover Control Workstation
    • Hardware Description
    • Functionality
• Overview of Rover Operations
  • Sample Sol 1 Scenario
  • Operations Timeline
  • RCW Outputs
  • Lander Processing of Rover Uploads
  • Rover Processing of Rover Uploads
  • Processing of Rover Telemetry
  • Rover Team Elements

• Meet cost, mass, and schedule requirements
• Minimize rover impact on Pathfinder project cost and risk
• Survive launch, cruise, landing
• Perform surface operations
  • complete technology experiments
  • carry out APXS rock and soil measurements
  • image lander
• Operating range:
  • operate primarily within 10 meters of lander
  • drive up to 100 meters on the Martian surface
  • be capable of operating beyond lander's horizon
• Lifetime:
  • complete 7-sol primary mission
  • be capable of extended mission up to 30 sols duration

To meet its mission objectives, the Microrover Flight Experiment (MFEX) rover must be able to:

• Communicate with lander to request commands and downlink telemetry
• Wakeup in response to either lander or on-board triggering
• Execute command sequences
• Unstow itself and drive down the lander ramps
• Traverse the surface of Mars while detecting and avoiding hazards
• Reach targets of interest as designated by Earth-based operators
• Maintain knowledge of its internal state
• Operate on-board experiments, including the APXS, MAE, and WAE
• Manage its limited available power
• Maintain its internal temperature within acceptable limits
• Perform a useful contingency mission if communications is lost
• Recover from command execution failures
• Continue to operate in the event of hardware degradation

• Rover Operating Characteristics:
  • a simple spacecraft
  • a serial machine - "Can't talk and chew gum at the same time"
  • operates in a non-deterministic environment - each step may yield unexpected results due to unknown terrain conditions
  • event driven
    • many commands (e.g., waypoint traverse) have significant (although bounded) uncertainty in execution time
    • command execution start time is determined by time of completion of previous command
    • rover sequences can be re-synched with lander activities by insertion of "Wait" commands at appropriate points in sequence

• Physical Characteristics:
  • Rover mass: ~11 Kg
  • LMRE mass: ~5 Kg
  • Rover dimensions (deployed): 63.0 cm (L) x 48 cm (W) x 28.0 cm (H)

• Mobility
  • Rover mobility hardware elements
  • Rocker-bogie chassis design
  • 6-wheel drive, 4-wheel steering
  • Rover speed ~0.4 meter/minute in nominal terrain
  • Vehicle capable of driving over obstacles 1.5 wheel diameters high
  • Rover can turn in place
  • Rover stowed flat during cruise to minimize volume
  • After release, rear wheels drive forward, raising rover to full height and locking rocker arms in deployed configuration

• Power
  • Rover power hardware elements
  • Primary power from GaAs solar panel
  • Backup power from non-rechargeable LiSiCl2 batteries
  • Primary mission objectives can be achieved if either (but not both!) power source is lost on landing
  • Only one of three batteries on-line until landing to prevent inadvertent battery drain, and maintain passivation layers

• Thermal
  • Rover thermal hardware elements
  • Internal electronics and batteries must remain in -40 degrees C to +40 degrees C (flight allowable) range
  • Warm Electronics Box (WEB) is actively heated during the day, cools overnight
  • Excess solar energy used to heat WEB to maximum allowable temperature
  • Thermal management process run between command executions or every 10 seconds when rover is idle
  • Heaters on batteries, modem, motors

• Imaging
  • 2 front B&W cameras used in conjunction with laser stripe projectors for hazard detection; also provide imaging
  • 1 rear color camera (rotated 90 degrees) used for imaging of APXS target area, rover tracks, and terrain
  • "Cameras" are CCDs clocked out by rover CPU
  • Auto-exposure capability
  • 4.9 to 1 BTC data compression available for B&W images (causes loss of color information if used with rear camera)

• Rover/Lander Communication
  • Rover communications hardware elements
  • Rover is the link master in half-duplex link (i.e., rover initiates all communications cycles)
  • Lander listens all day
  • Rover checks for sequence aborts before executing any command, and periodically when idle
  • Rover attempts several communications retries, if necessary, before buffering telemetry
  • Communications protocol uses error detection with retries (no error correction)

• Telemetry
  • Rover attempts to send telemetry at the completion of each command in a sequence
  • Telemetry buffered to RAM or EEPROM if lander not available (LIFO)
  • Buffered telemetry transmitted to lander at first opportunity

• Mission phases
  • Rover mission phases are:
    • prelaunch
    • cruise
    • prerelease
    • predeploy
    • primary
    • extended
    
    
  • Mission phase transitions governed by rover sensor information (e.g., accelerometer readings), current phase, and command
  • Phase used primarily to restrict rover actions (i.e., no use of actuators until rover has been released by the lander)
  • Also determines which contingency sequence to trigger, if needed

• Rover experiment hardware elements

  
  
  

• Alpha Proton X-ray Spectrometer (APXS)
  • determines elemental composition of rocks and soil
  • requires deployment of sensor head against rock or soil surface
  • data integration times up to 10 hours
  • APXS can be powered while rover is shut down
  • Rover cannot power both APXS and communications at once, but APXS needs significant periods of uninterrupted data collection
  • APXS operation while rover executes other commands necessitates temporary disabling of rover communications

• Material Adherence Experiment (MAE)
  • Provides measure of dust deposition on rover solar panel during surface mission
  • Clear glass dust cover operated using nitinol actuator; change in shorted solar cell current between open and closed dust cover states provides measure of transmittance of cover
  • Quartz Crystal Monitor (QCM): differential frequency of QCM changes as mass of material deposited on it increases

• Wheel Abrasion Experiment (WAE)
  • Abradable materials bonded to center wheel
  • During surface mission, material on wheel is worn away
  • Photocell is mounted above wheel
  • Change in reflectivity of segments of wheel provide measure of degree of abrasion

• Other Technology Experiments:
  • Terrain characterization
  • Basic soil mechanics
  • Sinkage
  • Thermal characterization
  • UHF link effectiveness
  • Vehicle performance
  • Dead reckoning and path reconstruction
  • Vision sensor performance

• Mission Experiments:
  • Imaging of lander
  • Lander damage assessment

• Rover navigation hardware elements

  
  
  

• Autonomous navigation requirement:
  • Due to communications constraints (light time delay, limited bandwidth, infrequent opportunities) and the need to respond in real-time to uncertain terrain conditions, the rover must be capable of autonomous navigation and hazard avoidance.

• Waypoint Traverse
  • Rover traverses to waypoints specified by human operator
  • Capability to reach desired target dependent on highly accurate designation of waypoints in IMP stereo images
  • Navigation occurs in the Surface Fixed Frame
  • Rover deviates from straight-line path between waypoints in response to detected hazards (rocks, drop-offs, excessive tilt, etc.)
  • Reflex-based control has no memory; no true onboard path planning
  • Rate gyro and odometry support dead reckoning
  • Daily updates of rover position/orientation provided from the ground based on analysis of IMP images, minimizing error accumulation

• Laser hazard detection system detects rocks, drop-offs, and slopes
  • Rover stops, captures image with selected lasers active
  • Given "flat-earth" assumption, laser stripe will be visible in known position on CCD scanline
  • Hazards cause laser stripe to slide along scanline (e.g., rock) or disappear (drop-off)
  • Repeating process with 5 lasers generates sparse "map" of ground in front of rover (5 x 5 grid of elevation points)
  • Hazard thresholds are empirically determined
  • Not a stereo imaging system

• Navigation safety features:
  • Heartbeat: the rover stops periodically and confirms contact with lander before continuing traverse; if communications fails, rover retreats short distance and attempts to reestablish contact
  • Lander avoidance: origin of navigation coordinate frame (lander position) is treated as a hazard, and avoided if necessary
  • Bumper contact sensors: Any obstacle which somehow eluded the laser hazard detection system will, in the worst case, trigger the bumper contact sensors, aborting the traverse
  • If waypoint destination is not reached by the time the command times out, remaining traverse is aborted

• Other traverse capabilities:
  • Find rock: Use hazard detection to approach an obstacle, rather than avoid it. Capability can be used to zero-in on APXS target rock.
  • Move: Drives rover without servoing or active hazard detection

• Command Loss (Earth/Lander Link)
  • Backup command sequence load stored on lander
  • Lander releases sequences to rover buffer in coordination with lander sequence execution
  • Rover executes sequences nominally, "unaware" of contingency situation
  • Approach allows for continued rover/lander activity coordination

• Rover/Lander Communications Link Failure
  • Rover transitions to contingency mission TBD hours after last successful receipt of command sequence upload
  • Appropriate on-board contingency sequence triggered, depending on rover mission phase when contingency state began
  • Rover assumes lander is still listening, sends telemetry without requiring ACK from lander
  • Any rover telemetry received by lander during rover contingency mission is sent to Earth as "unrecognized rover packets"
  • Rover continues to check for lander response, reverts to standard operation when command sequence received from lander

• Rover EEPROM corruption
  • Corrupted software will trigger "Rover Lite" command subset
  • Rover Lite resident in rad hard PROM only
  • Rover functionality limited in Rover Lite mode; ~50% of usual rover commands available
  • Allows for recovering via patching of new rover software load

• Device failures
  • Failure counters on all (80) devices used to assess device states
  • Failed sensors not used during rover operations
  • Counters decrement if device becomes operational
  • Devices can be forced "good" or "failed" by ground command

• Some onboard functional redundancy
  • accelerometers
  • use of steering potentiometers in place of gyro
  • etc.

• Rover LMRE hardware elements

  
  
  

• LMRE modem
  • provides lander side of rover/lander communications link
  • includes latchup detection/recovery circuitry
  • heater available to keep modem in operating temperature range

• LMRE antenna

  
  
  

• Rover mounting hardware
  • Rails to guide rover egress
  • Restraints and tie-down cables to fix stowed rover to petal

• Y-pin heater
  • Compensates for heat loss from rover through "coldfinger" heater after landing

• Ramps

• RCW hardware:
  • Silicon Graphics Crimson Reality Engine
  • 128 Mb Ram
  • 3 Gb disk
  • Spaceball 6-DOF input device
  • LCD shuttered goggles for stereo display
  • Second identical control station available as hot backup

• RCW software development environment:
  • C++
  • Open Inventor
  • Open GL
  • Builder Xcessory
  • Tools.h++
  • Motif 1.2.3

• Custom software implementing RCW features

  
  

• Interfaces
  • to GDS for uplink
  • to MIPL for IMP/rover images

• RCW features:
  • Graphical user interface for building rover command sequences
    

    
    

    

  • Integrated stereo display and graphics overlays for designation of rover navigation waypoints
    

  • 3D rover overlay for daily rover position updates
    
    

  • Constraint checking to prevent input of out-of-range values

• RCW inputs:
  • Rover Planning/Assessment Reports (RPR and RAR)
  • IMP image pairs provided by MIPL
  • Camera models associated with each image pair

• RCW outputs:
  • Rover Activity Sequence File (RASF): Command sequence sent to rover
  • Rover Activity Inspection Report (RAIF): File of comments to be inserted into SOE documents
  • Rover Assessment Report (RAR): Human-readable version of rover command sequence

• General rover telemetry processing not performed on RCW
  • Packet processing handled by Pathfinder GDS
  • Telemetry display on standard GDS workstations using DMD
  • Rover team provides DMD display definitions to GDS

• Waypoint Designation: Human operator places 3D cursor at the desired location in the stereo-rendered scene
  • Accurate waypoint designation requires a CAHV model of each camera, and knowledge of the alignment of the two cameras to each other
  • By selecting a waypoint in stereo, the operator effectively specifies that a pixel in the left camera view corresponds to the same location in the terrain as a particular pixel in the right camera view
  • Accurate camera models ensure that the computed location of a selected feature in the image pair corresponds well to its position on the surface of Mars
  • Camera models must be transformed as camera pointing changes (i.e., IMP pans/tilts)
  • Given the IMP's 1 mradian resolution, and the human operator's ability to designate to a fraction (~1/10) of a pixel, designations to within ~6 cm at 10 meters range are achievable

• Rover-related IMP Performance Issues
  • Successful designation assumes that IMP camera alignment and calibrations will not change
  • Camera toe-in for the IMP is effectively different for different filter sets; filter 5 is set of choice for rover ops
  • Errors in IMP pointing knowledge translate directly into designation errors (i.e., ~1 degree azimuth backlash uncertainty is equivalent to ~25 cm cross-range error at 10 meters range)
  • Can compensate for error if rover and target location both lie within the same image pair (relative designation then possible)
  • Image registration by MIPL can also eliminate relative uncertainty in pointing between images in a panorama of IMP images, allowing for accurate designation across multiple image pairs; this may take more time to process than is available on some sols

• Initial Conditions:
  • Rover release pyros have been fired
  • Ramps have been released
  • Rover state is nominal

• Sequence of activities:
  • unstow rover
  • drive down forward ramp onto surface
  • navigate to first waypoint
  • capture image of lander with color camera
  • perform MAE experiment
  • deploy APXS for soil data collection
  • capture rover operations images with forward cameras
  • collect APXS data for 1 hour
  • shutdown rover, but continue to collect APXS data overnight
  • awaken rover periodically overnight for health checks and transfer of APXS data
  • Set alarm clock to awaken rover at 8am local time in case solar wakeup fails

• RASF
  • submitted for uplink processing
  • A complete rover command sequence is viewed by Seqgen/Seqtran as the hex data field of a single ROVR_SEQNCE_LOAD command in the RASF
  • Seqgen performs no validation of rover command sequence itself
  • Seqgen does confirm that the rover sequence ID is a valid value given the NORMAL or EEPROM designation of the rover load

• RAIF
  • submitted for uplink processing
  • contains only comments associated with the rover commands in its associated RASF
  • Seqgen integrates RAIF comments into PEF file for inclusion in SOE

• RAR (Rover Assessment Report)
  • will be output in SOE editor format for convenient viewing
  • RPR (Rover Planning Report) is an Excel version of the RAR generated using custom application, independent of RCW

• NORMAL rover sequences
  • stored in RAM after receipt, not released into the lander's rover buffer
  • sequences identified by sequence ID between 1 and 32766
  • requires execution of lander command QUEUE_ROVER_LOAD with correct sequence ID to make sequence available to rover
  • deleted automatically after transmitted to rover

• EEPROM rover sequences
  • stored in EEPROM, not released to buffer
  • sequences identified by sequence ID between 32767 and 65535
  • requires execution of lander command QUEUE_ROVER_LOAD with correct sequence ID to make sequence available to rover
  • remain stored in EEPROM until explicitly deleted by lander command
  • may be reused (e.g., as part of Backup Mission Load)

• Appropriate rover-related lander commands must be included in associated lander upload
  • LMRE modem power must be on
  • modem should be power cycled periodically for latchup protection
  • IMP imaging commands necessary for updating rover position and generation of rover planning panorama
  • DPT table must include sufficient priority for rover telemetry

• Rover queries lander for new command sequence before executing next command in its currently active sequence
  • usually no sequence is available (nominally, expect to uplink one rover command sequence per sol)
  • if sequence is available, rover requests first frame
  • if first command is not "abort sequence" then rover cancels the communications session, leaving the new sequence on the lander
  • if first command is "abort sequence" then rover flushes its current command queue, and requests the rest of the new sequence

• Rover can process only one sequence at a time

  
  

• Rover performs sequence validation
  • CRC evaluated as sequence is loaded
  • individual commands validated before execution (e.g., number of parameters, range of parameter values)
  • check is performed to determine if sufficient power is available to execute command

• Lander Processing of Rover Telemetry
  • Rover packets placed into APID queues in same way as lander-generated packets
  • Rover telemetry is sorted into 12 APIDs
  • Lander has no visibility into telemetry data, except for header information

• Ground Processing of Rover Telemetry
  • GDS performs super commutation of rover packets (thanks!)
  • Rover telemetry will be fully integrated with Mars Pathfinder DMD system; rover engineering channels will be displayed as R-xxxx channels in a DMD "room" on any GDS workstation

• Rover Operations Team
  • Builds rover command sequences
  • Requests IMP imagery from IMP team
  • Interfaces with Experiment Team, Flight Engineers

• Rover Engineering Team
  • Evaluates rover engineering telemetry
  • Determines rover state
  • Develops recovery strategies
  • Interfaces primarily with rover ops team

• Rover Experiment Team
  • Represents rover technology experiments
  • Acts as part of overall experiment team
  • Interfaces with rover ops team and rover engineering team

Web page author: Andrew.H.Mishkin

All information on this site, including text and images describing the Rover, is copyright © 1996, Jet Propulsion Laboratory, California Institute of Technology and the National Aeronautics and Space Administration.